MR imaging is a medical imaging technique that uses an applied static magnetic field (B0) and radio frequency (RF) pulses to make images (e.g., via slices) of organs and structures inside the body. During MR imaging, the magnetic field causes magnetic field vectors of protons (typically in hydrogen atoms) to align with the magnetic field. The RF pulses cause the magnetic field vectors of the protons to be displaced (e.g., rotate) relative to the magnetic field and re-align with the magnetic field. A MR imaging scanner picks up signals from the protons in the body that result from magnetization field vectors re-aligning with the magnetic field. The signals may then be converted into images based on the location and strength of the incoming signals.
The magnetic field includes a number of locations in space, with each location having a corresponding magnitude and direction. Homogeneity or uniformity of the magnetic field refers to similarity of the magnitude and the direction of each location (e.g., three dimensional volume) of the magnetic field in space. A completely homogenous or uniform magnetic field has the same magnitude and the same direction for each location in space. The quality of MR imaging depends on the homogeneity of the magnetic field. The non-homogeneity of the main static magnetic field (B0) generated by an MR imaging system may lead to numerous imaging artifacts (e.g., banding artifacts) in MR imaging, such as steady-state free precession (SSFP) imaging.